One concern of rotorcraft designers is to reduce, to the extent practicable, the noise radiating from the rotor blades during flight operations. In particular, landing approaches which are characterized by a low speed, descending flight profile produce significant noise levels due to acoustic emissions known as Blade-Vortex Interaction (BVI). Insofar as such flight profile typically occurs at low altitude and over populated areas, BVI noise presents a primary technical issue which must be resolved to obtain community acceptance, and, more importantly, certification of newly-developed rotorcraft.
During typical rotorcraft flight operations, the rotor blades create a high velocity, low pressure field over the upper aerodynamic surface thereof and a low velocity, high pressure field over the lower aerodynamic surface. The pressure differential generates the necessary lift forces for flight operations but, additionally, effects the generation of vortices at the tips of the rotor blades. More specifically, the pressure differential engenders airflow circulation from the high pressure field to s the low pressure field to create a tip vortex. A tip vortex is shed from one rotor blade and impinges/interacts with a subsequent rotor blade as it rotates through the vortex field. The interaction of a tip vortex with a rotor blade induces impulsive airloading which creates an acoustic pressure wave that is the source of BVI noise.
BVI noise is generally not a concern in ascending or cruise flight modes inasmuch as the rotor disk, i.e., the plane defined by the rotor blades, moves away from the vortex wake. Consequently, the vortices are distally spaced from the rotor and do not significantly interact therewith. BV Interactions are most prevalent, however, during descent modes of operation, insofar as the downward velocity of the rotorcraft causes the rotor to fly into its wake thus interacting with multiple vortices.
The trajectory and core strength of vortices are difficult to predict; however, it may be generally stated that the vortices move downward in a spiral pattern as a function of the speed and flight attitude of the rotorcraft, the boundary conditions imposed by the fuselage, the turbulence of the atmosphere, and other factors such as the lift-time history of each rotor blade. Generally, BV Interactions which occur in the first quadrant of the rotor disk (0 degrees being positionally aft of the rotor shaft axis and along the longitudinal axis of the rotorcraft) generate strong BVI impulses due to the combined rotational velocity of the rotor blades and the forward flight velocity of the rotorcraft. Furthermore, the probability for strong interactions is intensified due to the high concentration and strength of vortices in this quadrant.
Another factor which influences the strength of the BV Interactions includes the orientation of the rotating vortex with respect to the impinging rotor blade. The orientation of the vortex is defined by the angle of intersection between the leading edge of the rotor blade and the centerline of the vortex, i.e., vortex core. When the orientation is substantially parallel with respect to the rotor blade leading edge, the circulatory flow of the vortex affects a large portion of the blade length, and, consequently, excites large impulsive pressure waves. Orientations which are substantially perpendicular or oblique to the leading edge produce relatively benign interactions by limiting the blade spanwise extent that is exposed to the circulatory flow.
Yet another factor which determines the strength of BVI encounters is the spatial separation between the rotating vortex and a passing/intersecting rotor blade. The spatial separation may be more accurately defined as the distance from the vortex core to a point on the leading edge of the rotor blade. Insofar as the airflow velocity at a point in the vortex field is a function of 1R, wherein R is the distance from the vortex core, it will be apparent that the airflow velocity, in theory, becomes infinite (.infin.) as R approaches zero (0) and diminishes at a precipitous rate as the distance R increases. Accordingly, when the spatial separation is small, e.g., less than the thickness of the rotor blade, the rotating vortex field will significantly impact the circulation about the rotor blade thereby producing large BVI impulses. Conversely, when the separation is larger, e.g., 5X the thickness of the rotor blade, the BVI impulse is substantially reduced due to the precipitous decline of vortex field velocity at the point of rotor blade intersection.
The rotorcraft designer, therefore, attempts, to the extent practicable taking into account, inter alia, weight, cost, performance, and system complexity, to incorporate elements into the rotor assembly that mitigate the BVI noise radiated therefrom. There are several different design options to mitigate BVI radiated noise. These approaches may be grouped into three broad categories, namely, passive systems, deployable passive systems and active systems.
Passive systems attempt to reduce BVI noise by favorably altering rotor blade geometry or rotor operating parameters. Examples of passive systems include selective tip shaping to diffuse or weaken the vortex. One design option involves a forward swept tip wherein the vortex is generated inboard of the tip, such inboard generated vortex being more diffuse, i.e., reduced in strength, than the tip vortex generated by a conventional rectangular tip. Another design configuration is a sub-wing tip wherein a sub-wing is attached to the rotor blade at the tip thereof to produce two weak, corotating vortices that mix downstream and diffuse due to viscous effects. Yet another design approach involves reducing tip speed, below about 675 ft/sec (206 m/sec), or increasing the number of blades to reduce blade loading, and, consequently, the strength of the tip vortex. These design options provide marginal improvement in mitigating BVI noise, e.g., on the order of about 2 to 5 dBa reduction and, furthermore, often degrade the overall operating efficiency of the rotor system. Furthermore, such design options may be difficult and/or costly to implement.
Deployable passive systems alter the rotor blade geometry in flight by deploying a noise reduction device during modes of operation which produce high levels of BVI noise. Examples of deployable passive systems include half-plow vortex generators which are disposed along the upper or lower surface of a rotor blade and are deployable when the rotorcraft is in a descending flight profile. Similar to the sub-wing tip discussed above, the half-plow vortex generators produce two or more vortices of reduced strength in an attempt to disrupt the formation of a single, more potent, tip vortex. While such vortex generation/deployable devices are generally effective in reducing BVI noise, performance penalties and/or mechanical complexities are impediments to the widespread acceptance of deployable passive systems.
Active systems effect noise control by continuously modifying the pitch or angle of attack of a rotor blade azimuthally about the rotational axis. This may be accomplished via selective control inputs by pitch control actuators or through blade mounted control surfaces which receive control inputs from a closed- or open-loop feedback control system. More specifically, the control system senses vibration/noise via a plurality of accelerometers/microphones and provides higher order control inputs to the control actuators/control surface to pitch the rotor blade at selected higher harmonic frequencies. The higher harmonic blade excursions effect vibration/noise reduction by influencing the trajectory and/or strength of the interfering vortex, and/or producing pressure waves that directly cancel the BVI pressure impulse. One example of an active system includes oscillating flaps disposed along the trailing edge of a rotor blade to provide a means of actively controlling the angle of attack of the rotor blade. Another example involves channeling air to the tip of a rotor blade and expelling such air to disrupt the formation of the tip vortex. Active systems are, perhaps, the most effective in mitigating the BVI noise when compared to passive and deployable passive systems, however, active systems are the most disadvantageous in terms of incurred weight penalties, complexity, reliability and related fail-safe issues.
A need, therefore, exists to provide a means to significantly reduce the BVI noise radiated from rotor systems which does not significantly degrade the operating efficiency thereof, e.g., lift capacity or power requirements, and does not significantly increase the weight, or mechanical complexity of the rotor system.